1. Field of the Invention
The present invention relates to a capacitive sensor system for detecting a physical amount by a small change of capacitance.
2. Description of the Related Art
A capacitive sensor generates an electrical signal in proportion to capacitance change caused by pressure or acceleration.
A prior art system for such a capacitive sensor includes a sensor capacitor, a reference capacitor, and a switched capacitor circuit connected to the sensor capacitor and the reference capacitor (see: Y. E. Park et al., "AN MOS SWITCHED-CAPACITOR READOUT AMPLIFIER FOR CAPACITIVE PRESSURE SENSORS", IEEE Custom Circuit Conf. pp. 380-384, 1983). In this prior art system, when a first driving voltage is applied to the sensor capacitor while applying ground voltage to the reference capacitor, the switched capacitor circuit is inactive. Then, when a second driving voltage is applied to the reference voltage while applying ground voltage to the sensor capacitor, the switched capacitor circuit is active to thereby generate a sensor voltage in response to pressure or acceleration applied to the sensor capacitor. This will be explained later in detail.
In the above-described prior art system, however, when the switched capacitor circuit is transferred from an inactive state to an active state, a charge stored in a feedback control switch transistor of the switched capacitor circuit appears in the output thereof as an offset voltage. This offset voltage also depends upon ambient temperature, so that a temperature drift is also generated.